SLI is dead. nVidia have dropped support for it on all their consumer cards except the 3090. You’re not likely to see support for it in MSFS ever unless nVidia and AMD do a 180 course reversal and make multiple GPUs a thing again. And they’re not likely to do that.
Outside of professional rendering workloads or AI applications, there’s absolutely zero use for multiple bridged GPUs in this day and age nor going into the immediate future.